A ball screw device according to Patent Document 1 is disclosed as this type of conventional ball screw device. The ball screw device disclosed in Patent Document 1 results from dividing a resin rotational part (not shown) into two parts and arranging them such that the centerlines of recirculating paths within legs of the rotational part fitted to recirculation holes of a nut approximately coincide tangentially (approximately tangential to a spiral raceway) and in a lead angle direction of both grooves so as to provide smooth scooping up of balls (rolling elements).
However, since the ball screw device disclosed in Patent Document 1 has the rotational part divided into two parts radially along the centerlines of the ball recirculating paths, even a tongue portion for scooping up the balls is divided, and there is concern that the strength of the tongue portion will be reduced.
Therefore, the ball screw device of Patent Document 2, which avoids such division of the tongue portion, has been disclosed.
FIG. 6 is a diagram illustrative of a configuration of the conventional ball screw device disclosed in Patent Document 2, where FIG. 6(a) is a top view and FIG. 6(b) is a view of a rotational part when seen from the axial direction of a screw shaft. As shown in FIGS. 6(a) and 6(b), a ball screw device 101 disclosed in Patent Document 2 has a nut 120 including a spiral groove (not shown) corresponding to an inner surface groove 111 fitted to a screw shaft 110 including a spiral external surface groove 111. Multiple balls (not shown) are loaded in a rollable fashion along a spiral raceway K between the grooves 111 and the groove of the nut 120.
Moreover, a flat surface 120a is formed on the peripheral surface of the nut 120, and two pairs of recirculation holes 122 passing to the spiral raceway K are bored in the flat surface 120a. A recirculating part 140 is established so that both ends are fitted to the pairs of recirculation holes 122, forming a ball recirculating path where the balls are lead to the outside of the nut 120 through one of the recirculation holes 122 and then back to the spiral track K through the other of the recirculation holes 122. The recirculating part 140 is comprised by a pair of legs 141, each having a tongue portion 144 for scooping up the balls that roll along the spiral raceway K on ends thereof in a direction approximately tangential to the spiral raceway K, and a main body 142 connecting the pair of legs 141. Furthermore, the recirculating part 140 is formed including the pair of legs 141 and the main body 142 connecting the pair of legs 141, the interior made as a ball recirculating path R, and divided bodies 140a, which are divided in two point symmetrically along the ball recirculating path R, fitted by divided faces. Circulating grooves 143 constituting the ball recirculating path R are formed in the divided faces of the respective divided bodies 140a. 
The circulating grooves 143 constituting the ball recirculating path R each includes a first curvature portion R1, a second curvature portion R2, a third curvature portion R3, and a fourth curvature portion R4. The first curvature portion R1 is a portion leading a ball, which is scooped up from the spiral raceway K by the tongue 144 of one leg 141 of the pair of legs 141 and 141, to the outside of the nut 120 in a direction having a predetermined angle or approximately coinciding with the axial direction of the recirculation hole 122. The second curvature portion R2 is a portion leading the ball having passed the first curvature portion R1 to the other leg 141. The third curvature portion R3 is a portion leading the ball having passed the second curvature portion R2 to the inside of the other leg 141 via the main body 142. The fourth curvature portion R4 is a portion leading the ball having passed the third curvature portion R3 to the position of the tongue portion 144 of the other leg 141.